This invention relates to direct-heated cathode structures which support the direct-heated cathode in a cathode-ray tube and to methods for the fabrication of the direct-heated cathode structure.
A direct-heated cathode structure is an element for use in an electron gun wherein a cathode pellet is heated by a heating element and emits thermions; and the structure is classified into a direct-heated type and an indirect-heated type according to the relative position of the cathode pellet.
In the above statement, the direct-heated type has its cathode pellet itself placed on the filament and the indirect-heated type has its cathode pellet placed on a cap which surrounds the filament thus the cathode pellet contacts the heating element indirectly.
While the direct-heated type and the indirect-heated type have different merits and deficiencies according to their structures respectively, the former when it is compared with the latter is characterized by its simplicity in structure, its low power consumption rate and the fact that the picture appears on the cathode-ray tube screen quickly. It is employed in electric viewfinders for portable small-sized televisions or video cameras.
On the contrary, one of the deficiencies of the direct-heated type is in that the interspace between the first grid and the cathode pellet in the direct-heated type cathode structure varies when the filament emits heat producing tension changes in the filament because of thermal expansion.
The abovementioned thermal expansion which is produced not only in the filament but also in the filament supporting structure, causes dimensional changes in the interspace between the filament and the supporting structure, and in their respective disposition; and these dimensional changes affect the cut-off voltage thus deteriorating the white balance.
As is described in U.S. Pat. No. 3,633,062 as a prior art in order to solve these problems originated from the thermal expansion, a material with low thermal expansion coefficient such as ceramic is adopted. At the same time, the changes of tension in the filament is to be absorbed by supporting both ends of the filament with spring reeds.
FIG. 4 illustrates a general structure of the direct-heated cathode structure with a ceramic base.
The ceramic base 1 has penetration hole 1' at both sides whereto supporting bars 2 are inserted and bonded; springs 3 with their ends bend outwardly are welded to said bars 2 at their upper ends; and a filament 4 is built in between upper ends of said springs 3.
Above mentioned supporting bars 2, as they act as electrical conduit for impressing power to the filament, are made of conductive material and they are inserted through the penetration holes 1' and bonded to the ceramic base 1 by means of frit 5 because welding metal and ceramic together is not feasible.
The cathode pellet 6 is bonded on a base metal formed at the upper center of the filament maintaining some interspace from the first grid 7.
According to this direct-heated cathode structure, the filament 4 when it's impressed by suitable voltage produces heat from 700 Txc to 800 Txc normally thus making the cathode pellet 6 emit thermions toward the first grid 7. As the change in the filament length caused by the thermal expansion during this process will be absorbed by springs 3 at both ends of the filament, the interspace between the cathode pellet 6 and the first grid 7, and their respective disposition seldom experience changes thus the cathode pellet is maintained to face always the center of the first grid.
However, in the above mentioned direct-heated cathode structure, it is not only difficult to maintain the accuracy in assembling the structure but also the number of processes required increases because supporting bars 2 are fixed by sintering to the ceramic base 1 by means of frit 5, and furthermore, a glassy laminar insulator which interrupts the spot welding of the conductive tapes 2' onto supporting bars 2 will be sintered over the surface of the supporting bars if the frit runs down the surface.
In addition to the above, supporting bars 2 will be buckled by the pressure applied during the welding of conductive tapes 2' to the lower ends of supporting bars 2 and as shown by chained line in FIG. 4, there will be some deformation in the respective disposition between supporting bars 2 and springs 3.
Because of the above mentioned deformation, thermal vibration will occur in the filament 4 at the beginning of heating and it is the main reason for the picture noises.
The present invention is directed to provide a direct-heated cathode structure with its supporting bars attached to the ceramic base by mechanical means and an adequate fabrication method for the same.
Also the present invention is directed to provide a direct-heated cathode structure which suffers no deformation in the respective disposition state between its supporting bars and the filament by confining the deformation of the supporting bars that may happen to its lower end portion only during the welding of conductive tape to the supporting bar.